Self-gating synchronizing circuit



Npv. 20, 1956 A. M. LEVINE ETAL SELF-GATING SYNCHRONIZING CIRCUIT FiledOct. 2, 1953 INVENTORS ATTORNEY United States Patent 2,771,507SELF-GATIN G SYNCHRONIZING CIRCUIT Arnold M. Levine, River Edge, andHarry Altman, Bellevllle, N. J., assignors to International Telephoneand Telegraph Corporation, Nutley, N. J., a corporation of Maryland Thisinvention relates to improvements in synchronizing circuits such as, forexample, those used in television receivers and more particularly itrelates to synchronizing circuits used in television receivers in whichlocal means are employed to generate a synchronizing pulse in responseto the transmitted synchronizing signals.

With the expansion of television station coverage and thesignal-to-noise improvement in television receivers, many suburban areaswhich heretofore were unable to receive television transmissions are nowable to view television. However, in many of these regions it isextremely difficult to obtain satisfactory synchronization, especiallyunder weak and noisy signal conditions because the television receiversare extremely sensitive to any decay in the quality of the transmittedsynchronizing signal. .In most television receivers it is common toemploy a synchronous oscillator, such as one of the blocking oscillatorvariety, responsive to the transmitted synhcronizing signals to obtainpulses of suflicient energy content to initiate synchronization withinthe receiver. Such a device may be operated at a frequency which isappreciably different from that of the occurrence of the transmittedsynchronizing signals. However, by virtue of the introduction of thedetected synchronizing signal, the oscillator is triggered andgeneratessynhcronizing pulses of considerably greater energy contentthan the' detected synchronous signals, but the generated pulses recurat equal time intervals with the received synchronizing signals. Theselocally generated pulses are far more effective than the receivedsignals in synchronizing the television receiver because of their higherenergy content. Of course, it is necessary to guard against thetriggering of the oscillator by noise or interference signals andyet'make sure that the oscillator will be responsive to weakly receivedsynchronizing signals.

One disadvantage in such a system derives from the fact that the freeperiod of the blocking oscillator may tend to vary during operation tosuch an extent that the device is no longer capable of beingsynchronized or locked in with the received synchronizing signals so asto generate a pulse signal corresponding to each and every receivedsignal. Alternately, the received synchronizing signal may be so smallor decayed that it is ineffective in initiating or triggering theoscillator action.

One of the objects of this invention therefore is to provide asynchronizing oscillator having a greater immunity to spurious noisesignals.

Another object of this invention is to provide a synchronizingoscillator having a high stability so as not to be affected byvariations in the supply voltage or input signals.

A further object of this invention is to provide a synchronizingoscillator wherein the received synchronous signals effect a change inthe oscillator sensitivity curve so as to cut down the time regionduring which the oscillator is susceptible to being fired.

A feature of this invention is the use of a highly stable blockingoscillator which is triggered by the received synchronizing signal. Thereceived signal is also utilized to determine a gating period duringwhich the oscillator is susceptible of being fired. The receivedsynchronizing signal is coupled to the grid of an electron dischargedevice having a time constant in its grid circuit. The electrondischarge device functions as a limiter-amplifier whose output isamplified in a second electron discharge device and is inductivelycoupled back to the grid of the first electron discharge along with thesynchronizing signals.

Another feature of this invention is the self gating of a blockingoscillator due to coupling the output of a limiter-amplifier electrondischarge device to an amplifier whose output is regeneratively coupledback to the limiteramplifier whereby th limiter-amplifier is capable offiring only during that portion of time during which the output of theamplifier is coupled into it.

The above-mentioned and other features and objects of this inventionwill become more apparent by reference to the following descriptiontaken in conjunction with the accompanying drawings, in which:

Fig. 1 is a circuit diagram of one embodiment of a synchronousoscillator of this invention; and

Fig. 2 illustrates series of curves helpful in the explanation of thcircuit diagram shown in Fig. 1.

Referring to Fig. 1 of the drawing, a synchronizing circuit inaccordance with the principles of this invention is shown therein foruse as a vertical synchronizing oscillator in a television receiver. Thereceived television signal is detected and the vertical synchronizingsignals are coupled to the input terminal 1 to function as a triggersignal for the synchronous oscillator. The input terminal 1 is coupledto capacitor 2, across the resistancecapacitance (R-C) time constantnetwork 4 and through one winding 5a of transformer 5 to the grid 6 ofthe electron discharge device 7 whose cathode 8 is grounded. The plate 9of tube 7 is coupled by capacitance 11 to the cathode 12 of electrondischarge device 13 having a grid 14 which is grounded. The plate 15 oftube 13'is coupled through the second winding 5b of transformer 5 to asource of positive bias voltage.

Referring to Fig. 2, curve A, a typical grid voltage-time wave shapefound in the circuit of Fig. 1 is shown for a free-running condition,that is when there are no trigger signals coupled to input terminal 1.The exponential portion 20 of curve A is due to the discharg ofcapacitor 3 in the time constant network 4 and the voltage waveformwhich is dependent on the time constant varies from a level well belowcut-off up to the cut-off voltage level shown by dotted line 21. As thedischarge of capacitor 3 reaches the cut-off level 21 the bias on thegrid 6 causes tube 7 to conduct as shown by pulse 22 of curve A, Fig. 2.When the tube 7 conducts, its plate output 24, curve B, causes tube 13to conduct and cause a similar pulse 25, curve C, of greater amplitudeto surge through the winding 5b of transformer 5. By regenerative actionthrough transformer winder 5a, the grid circuit of tube 7 is caused todraw current thereby recharging capacitor 3 negatively as indicated bythe portion 23 of curve A. As soon as curve portion 23 drops belowcut-off 21 the tube 7 no longer conducts as indicated by its plateoutput 24. The surge applied to transformer 5 causes a series of dampedoscillations to occur in the grid circuit as indicated at 26, curve A.

In accordance with th principle of our invention we adjust the timeconstant of this RC circuit 4 for a free running frequency slightlylower than the recurrent frequency of the synchronizing pulse signals27, curve D. As shown in curve E, the tube 7 is caused to conductearlier than it would during free running operation, and the triggeringpulse 27 in addition causes th tube 7 to draw more current as indicatedat 28. This results in a corresponding increase in the pulse output oftube 13 thereby increasing the amplitude of the initial oscillations ofthe transformer as indicated at 29. Due to this additional energyintroduced into the grid circuit of tube 7, grid 6 draws more currentthan it does when in the free running condition, and this results in agreater charge on capacitor 3, thus increasing the negative bias on grid6 as illustrated by portion 30, curve E, in comparison with portion 20for free running operation. As capacitor 3 discharges, the exponentialcurve 30 does not approach as near to the cut-off level by the occurrencof the next triggering pulse 27a as it would in the free runningcondition. As stated above, output of tube 7 is coupled to the cathode12 of tube 13 where it is amplified and the output of tube 13 isregeneratively coupled to th grid circuit of tube 7 through thetransformer increasing the surge when tube 7 conducts in response to atriggering pulse. Due to the increased charge in capacitor 3 causing theexponential curve 30 to be further from the cut-off level, it isapparent that under these conditions the circuit of this invention canonly be triggered prior to a synchronizing pulse by a noise pulse ofsufiicient amplitude to raise the level from the exponential curve 30 toth cut-off level 21. Effectively, the grid voltage bias curve has beenlowered in relation to the time of the input synchronizing pulsesthereby reducing the chance of the oscillator being triggered by noiseenergy occurring prior to the trigger pulse.

Should the synchronizing signal be momentarily lost, the oscillator willimmediately revert back to its free running condition, thus allowing therecovery time of the oscillator to be raised and resulting in a fastacting system. It has been assumed that proper limiting exists prior toinput terminals 1 and this is usually true because the peak noise neverexceeds the peak of the synchronous pulses after limiting. Thus, ourimproved synchronous oscillator is found to reduce th noisesusceptibility region to a minimum, and due to its greater stability, itis independent of changes in the supply voltage and filament voltage.Due to the large regenerative pulse gain, the repetition frequency isvery high and becomes to a great extent independent of the trigger pulseshape yielding a good interlac characteristic which is substantiallyindependent of the hold control setting.

While we have described above the principles of our invention inconnection with specific apparatus, it is to be clearly understood thatthis description is made only by way of example and not as a limitationto the scope of our invention as set forth in the objects thereof and inthe accompanying claims.

We claim:

1. In a television system having synchronizing signals, a blockingoscillator circuit to generate pulse signals having a rate of occurrencecomparable to said synchronizing signals comprising a first electrondischarge device having at least a cathode, an anode and a grid, acircuit for said grid, a time constant circuit having capacitive andresistive elements coupled to said grid circuit, means to couple saidsynchronizing signals to said grid circuit, a second electron dischargedevice having at least a cathode and an anode, means to couple theoutput from the anode of said first electron discharge device to thecathode of said second electron discharge device and inductive means tocouple the output of the anode of said second electron discharge deviceto said grid circuit of said first electron discharge device.

2. In a television system having synchronizing signals, means togenerate pulse signals having a rate of occurrence comparable to saidsynchronizing signals comprising a first electron discharge devicehaving at least a cathode, an anode and a grid, a circuit for said grid,21 time constant circuit have capacitive and resistive elements coupledto said grid circuit, means to couple said synchronizing signals to saidgrid circuit, a second electron discharge device having at least acathode and an anode, means to couple the output from the anode of saidfirst electron discharge device to the cathode of said second electrondischarge device and means to couple the output of the anode of saidsecond electron discharge device to said grid circuit of said firstelectron discharge device, said last named means including a couplingtransformer having one winding coupled to the anode of said secondelectron discharg device and a second winding coupled to said gridcircuit.

3. In a television system having synchronizing signals, means togenerate pulse signals having a rate of occurrence comparable to saidsynchronized signals comprising a transformer having a first and secondwinding, a first electron discharge device having at least a cathode, ananode and a grid, said cathode being coupled to ground potential, acircuit for said grid including said first winding, a time constantnetwork having at least a capacitive and resistive element coupled inparallel between said grid circuit and ground potential, a source ofinput synchronizing signals coupled to said grid circuit, a secondelectron discharge device having an anode, a cathode and grid, said gridbeing coupled to ground potential, the anode of said first electrondischarge device being coupled to the cathode of said second device, aresistor, said resistor being coupled between the cathode of said seconddevice and ground potential, an output terminal coupled to the anodeoutput of said second electron discharge device and said secondwindingof said transformer being coupled to the anode of said second device.

4. In a television system having synchronizing signals, a pulsegenerating circuit of the blocking oscillator type to generat pulsesignals having a rate of occurrence comparable to said synchronizingsignals comprising an electron discharge device including a gridcircuit, a time constant circuit coupled in said grid circuit, means tocouple said synchronizing signals to said grid circuit to trigger saiddevice for conduction, means to amplify the signal output of saidelectron discharge device and conductive coupling means toregeneratively couple said amplified signal output to said grid circuitand thereby minimize the action of spurious voice pulses on theoperation of said pulse generating circuit, the means to amplify thesignal output of said electron discharge device including a secondelectron discharge device having at least a cathode, an anode and agrounded grid, means to couple the output from said first-mentionedelectron discharge device to the cathode of said second electrondischarge device and a transformer to couple the output of said secondelectron discharge device to said grid circuit, said transformer havingone winding coupled in the anode circuit of said second electrondischarge device and a second winding in the grid circuit of said firstmentioned electron discharge device.

References Cited in the file of this patent UNITED STATES PATENTS2,585,930 Gruen Feb. 19, 1952

